1. Field of the Invention
The present invention relates to solder balls.
2. Description of Related Art
In recent years, electronic devices have increased in communication speed and capacity, and become multifunctional and portable at a rapid pace. The advance of surface mount technology (ball grid array (BGA)/chip scale package (CSP)) of a package for a semiconductor integrated circuit or the like has improved fine wiring and multiple pins, thus leading to increase in communication speed and portability.
Unlike conventional bonding methods using a metal lead frame, such as a quad flat package (QFP) or a small outline package (SOP), the BGA/CSP technology involves forming bumps using solder balls, and then directly soldering a package onto a printed wiring board via the bumps in many cases.
A ball mount type bump formation technique is in a mainstream of the bump formation techniques in which solder balls are mechanically mounted on a package substrate by a ball mounter, and then reflowed thereby to form solder joints. In the ball mount, the solder balls are vacuum-sucked by a dedicated jig, and a flux is applied to the balls or the substrate. Then, the solder balls are mounted on the substrate. After appearance inspection, a reflow process is performed.
In such a ball mount process, the balls to be mounted may be micro-bonded together and not sucked by the mounting jig, or the balls may be attached to the mounting jig at the same time to cause a defective condition or the like. The micro-bonding of the solder balls is caused by triboelectric charging.
Detection of the solder balls is performed using an optical detector after mounting the solder balls. The optical detection generally involves bringing the ball into focus while the surface of the ball is shiny. The element Sn or Pb will be oxidized to have its color changed, which promotes the etiolation of the surface of the ball. The progress of oxidation of the solder ball, and of the etiolation of the surface may disadvantageously cause malfunction of the optical detector for detection of the mounted balls, which results in a decrease in yield.
A technique for prevention of the color change of the solder balls is disclosed in Japanese Patent No. 3503523. In the technique, an aliphatic hydrocarbon lubricant, such as stearic acid or stearic acid amide, or a higher aliphatic amide lubricant is diluted with a solvent, and then the solder balls are immersed in the lubricant, so that the solder balls have the surfaces thereof coated therewith. This prevents friction and triboelectric charging of the solder balls due to swinging or vibration during transportation or mounting of the solder balls, thereby to prevent the change in color. The surface treatment technique has effects of improving the lack of the bonding strength and contamination of an area near a position for a solder bump formation.
The above patent document, however, does not specifically describe a surface treatment agent other than the stearic acid and stearic acid amide. The surface treatment using such kinds of limited organic compounds cannot sufficiently prevent the change in color of the solder balls, and has difficulty in completely preventing mounting errors from being caused by the micro-bonding between the solder balls due to the triboelectric charging as described above. Further, in order to achieve mounting at high density, a pitch of electrodes on a substrate has become fine, and the diameter of a solder ball has decreased, which may cause problems, including micro-bonding and the like. From the viewpoint of background described above, it is difficult for the related art to address the foregoing problems.
In recent years, a lead-free solder ball has been in the mainstream owing to a restriction of hazardous substances (RoHS) command that went into effect in 2006, instead of the conventional Sn—Pb based solder. Alloy components for the solder ball in use include an Sn—Ag based alloy, an Sn—Ag—Cu based alloy and the like. Decrease in amount of Ag of the alloy component, or use of Sn—Zn based component has been recently studied because of a rise in the price of rare metal.
The Pb-free solder composition has poor wettability as compared with the Sn—Pb based solder. A high reflow temperature promotes the oxidation of the solder balls to reduce the wettability, which causes the problems associated with the reliability of bonding.
Thus, a process for coating the surface of the solder ball requires a surface treatment which is adapted to suppress the friction of the solder ball for preventing the change in color of the solder balls, and to deoxidize an oxide film on the surface of the solder ball, thereby removing the oxide film.
Accordingly, it is an object of the present invention to improve the yield of electronic devices in a ball mounting process by preventing the micro-bonding and change in color of the solder balls, and to improve the bonding reliability of the solder balls by reducing an oxide film on the surface of the solder ball.